A semiconductor media that could be printed under ambient conditions by a roll-to-roll process on plastic substrates to fabricate photovoltaic devices (PVs) would greatly reduce the cost of solar cell manufacturing. One approach is to create “paints” or “inks” of nanocrystals that can be deposited by solution deposition processes like casting, dip-coating or spraying. Recently, our group demonstrated that copper indium diselenide (CIS) nanocrystals can be synthesized and dispersed in organic solvents and then used to fabricate functioning PVs without the need for high temperature annealing or vacuum processing of the absorber layer. Unfortunately, the power conversion efficiency of these kinds of devices is still too low for commercialization, largely because of the losses in the film due to the high density of interfaces between neighboring nanocrystals. One approach to eliminating these interfaces, while retaining the printability of the media is to synthesize nanorods or nanowires that can be dispersed and printed. Here, we report the synthesis of CIS nanowires by solution-liquid-solid (SLS) and supercritical fluid-liquid-solid (SFLS) growth. Nanocrystals of bismuth are used as crystallization seeds of the nanowires in either a hot high-boiling solvent, or in an organic solvent under pressure at temperatures well above the critical point of the solvent. Significant quantities of nanowires can be synthesized using these approaches. Prototype PVs have been fabricated by spray-deposition of the nanowires. We also demonstrate the formation of a nanowire paper, or non-woven fabric, with the potential to serve as a new type of free-standing PV material.
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